2. Objectives
•To know the anatomy of Anaesthesia machine
and safety
•To know inhaled anesthetics delivery system
3. Key Points
• If there is any possibility that the workstation or the breathing circuit is a
potential cause of difficulty with ventilation or oxygenation, switching to a
self inflating resuscitation bag is an appropriate decision.
• - Miller’s Anesthesia.
Ventilate and oxygenate first – troubleshoot later.
The most important part of the preuse anesthesia workstation checkout
procedure is verifying the presence of a self-inflating resuscitation bag.
4.
5.
6.
7. Basic components of Gas Supply System
Pressure Systems
• High
• Confined to cylinders and cylinder primary pressure regulators
• O2 – 2200 psig to 45psig
• N2O – 750 psig to 45 psig
•Intermediate
- Begins at pressure regulated cylinder supply source at 45 psig, include the
pipeline source of 50-55 psig and extends to flow control valves
•Low
• Extends from the flow control valves to the common as outlet
8. Measurement of pressure
Psi = pounds per square inch
Psig = pounds per square inch gauge
(Difference between measured pressure and surrounding atm pressure)
Psia = pounds per square inch absolute
(Based on a reference point of ‘0’ pressure=perfect vacuum)
Psia = psig + local atm pressure
100 kPa = 1000 mbar = 760 mm of Hg = 1030 cm of H2O
= 14.7 psi = 1atmosphere
11. What is vaporizer?
• closed container that converts liquid, that exist as a liquid in room
temperature and atmospheric pressure, into a vapor.
12. Why we need a vaporizer?
• Add a certain amount of this vapor in precisely determined
concentrations over a wide range of temperatures, pressures and
carrier gas flow rates.
13. • Vapor - phase of an agent that is normally liquid at room temperature
and atmospheric pressure .
14. Critical temperature of isoflurane is about 200°C
• Earth’s room temp.
• 21°C
• Lower than critical temp
• Isoflurane vapour
• Venus temp.
• 500 °C
• higher than critical temp
• isoflurane gas
19. • When we dial a high anaesthetic concentration requirement, the splitting
valve sends more fresh gas via the vaporising chamber.
• Similarly, when we dial a low anaesthetic concentration requirement, the
splitting valve sends less fresh gas via the vaporising chamber.
• Contemporary anesthesia vaporizers for halothane, isoflurane, enflurane and
sevoflurane are classified as
variable-bypass, flow-over, temperature-compensated, agent-specific, out-of-
circuit vaporizers (also called concentration calibrated, automatic plenum, dial-
controlled)
• Desflurane vaporizer (Ohmeda Tec 6) is of different design
21. • Agent concentration is controlled by a dial calibrated volume percent.
• Volume vaporized- typically 200ml vapor per ml of liquid anesthetic.
22.
23.
24.
25. • 1 ml of liquid anesthetic ~ 200 ml of anesthetic vapour ………. Equation -1
Or, 100 ml of isoflurane = 200 * 100 ml of isoflurane vapour
= 20 liters of Iso gas
If we administer 1% Isoflurane with 2 l/min (2000ml/min) FGF
1% of 2000 ml = 20 ml of gas each min
i.e in 1 hour we have 20 *60 ml = 1200ml of gas each hour.
From equation 1
Therefore, we use 1200/200 = 6 ml of liquid isoflurane each hour.
Simplified formula:
ml liquid used/hour = 2 x% x FGF
26. Measured flow
• usually oxygen is used to pick up anesthetic vapor.
• A set of separate oxygen flowmeter to pass to the vaporizer
from which vapor at its SVP emerges.
28. Flow-over
• The gas channeled to the vaporizing chamber flows over the liquid
anesthetic multiple times to become saturated with vapor
• A series of baffles repeatedly redirect the mixed gas flow onto the
surface of the liquid anesthetic agent to achieve full saturation.
29. Bubble-through
• Carrier gas is bubbled through the volatile liquid usually by means
of a sintered disc.
• increase the gas-liquid interface.
• Certain vaporisers (e.g. "Copper Kettle") use bubbles to increase the
surface area for vaporisation. In these, some of the fresh gas flow is
bubbled through a disk made out of a special material (sintered
disk) that is very porous. The disk is submerged into the anaesthetic
agent and when fresh gas is sent through it, a large number of tiny
bubbles form. The tiny bubbles of fresh gas have a very large total
surface and thus become fully saturated with vapor efficiently.
30. Injection
• Vapor concentration is controlled by injecting a known amount of
liquid anesthetic into a known volume of gas .
• The anesthetic agent is delivered into the gas stream through a fine
nozzle. The rate of delivery depends on the pressure difference P1
to P2 across the nozzle which is adjusted by the throttle valve
• If flow through the vaporizer is increased, the pressure across the
valve is increased and so more anesthetic is delivered to maintain
the same concentration
• Therefore, the vaporizer remains accurate despite changes in flow
34. Characteristics of an ideal vaporizer
1. Performance not affected by changes in
fresh gas flow
volume of the liquid agent,
ambient temperature and pressure,
pressure fluctuation due to respiration
Tilting and tipping
2. Low resistance to flow
3. Light weight with small liquid requirement
4. Economical and safety in use with minimal servicing requirements
5. Corrosion and solvent-resistant construction
35. • If an ideal vaporizer existed, with a fixed dial setting its output would be
constant regardless of varied flow rates, temperature, backpressure,
and carrier gases.
• Designing such a vaporizer is difficult because as ambient conditions
change, the physical properties of gases and vaporizers themselves can
change.
• Contemporary vaporizers approach ideal but still have some limitations
36. Factors influencing the Vaporizer Output
A. Flow Rate
B. Temperature
C. Pressure
D. Composition of Carrier Gas
37. Flow Rate
• At low flow rates (<250 ml/min), the output of variable-bypass vaporizers is
less than the dial setting
• At very high flow rates (e.g. 15 L/min), the output is also less .
38. • With a fixed dial setting, vaporizer output can vary with the rate of gas
flowing through the vaporizer.
• This variation is particularly notable at the extremes of flow rates.
• The output of all variable-bypass vaporizers is less than the dial setting at
low flow rates (<250 mL/min) because of the relatively high density of
volatile inhaled anesthetics.
• Insufficient turbulence is generated in the vaporizing chamber at low flow
rates to upwardly advance the vapor molecules.
• At extremely high flow rates, such as 15 L/min, the output of most
variable-bypass vaporizers is less than the dial setting.
• This discrepancy is attributed to incomplete mixing and failure to saturate
the carrier gas in the vaporizing chamber.
• In addition, the resistance characteristics of the bypass chamber and the
vaporizing chamber can vary as flow increases.
• These variations can result in decreased vapor output concentration
39. Surface area
Surface area of the liquid gas interface:
• greater the surface area, more will be the vaporization .
• Bubble through > flow over
41. Thermocompensation
The metal helps to minimize the temperature drop by two
ways.
1. efficiently transfer heat from the surrounding air into the
anesthetic agent.
42. 2.Metal acts like a 'heat store'. It 'absorbs' heat till its temperature
equals the temperature of the surrounding air.
48. Effect of intermittent back Pressure
The 'pumping effect' increases the delivered concentration of
anesthetic agent.
49. Greatest increase in pumping effect occurs at:-
1. Low fresh gas flow rates
2. Low concentration dial settings
3. Small amount of liquid agent in vaporizer sump
4. Large and rapid changes in pressure
50. Measures to decrease Pumping Effect
• Keep the vaporizing chamber small or increasing the size of bypass chamber.
• LONG INLET TUBE: the extra gas containing vapor expands into the long inlet
tube and doesn't reach the 'by pass' channel.
51. Effects of altered barometric pressure
Low Atmospheric Pressure
• Decreased barometric pressure will affect a concentration calibrated vaporizer by
altering the splitting ratio
• The high-resistance pathway through the vaporizing chamber offers less resistance under
hypobaric conditions, increasing vaporizer output
• The delivered partial pressure and volume % increases.
• Closer the vapor pressure is to the barometric pressure, greater the effect.
52. Effects of altered barometric pressure
For eg, at 630mm hg,
if isoflurane is set at 1%,
the actual output = 1(760/630)=1.2%
53. Effects of altered barometric pressure
• Set concentration : 1%
• Delivered concentration: 1.2%
• Is this a worry???
• Partial pressure of 1% isoflurane at 760mm hg is 7.6mm hg
• Partial pressure of 1.2% isoflurane at 630mm hg is 7.6mm hg
54. High Atmospheric Pressures – Hyperbaric Chamber
• changes in the density of gases cause more resistance to flow
through the vaporizing chamber and a decrease in vaporizer output
Partial pressure of the volatile anesthetic agent in the central
nervous system, not its concentration, is responsible for the
anesthetic effect.
56. • Desflurane has a very low boiling point (about 23 degrees Centigrade) and
even at room temperature, has an high vapor pressure.
• Desflurane is an extremely volatile anesthetic with a SVP of 669 mmHg at 20C
and boiling point of 22.8C
• These properties make it challenging when it comes to vaporization and
production of controlled concentrations of vapor
• If conventional variable bypass vaporizers are filled with desflurane, it will boil
at 22.8C in the vaporizing chamber leading to uncontrolled output from the
vaporizer, and overdosing
• Liquid desflurane in heated is a chamber (the sump) to 39C to produce vapor
under pressure ( 1500 mmHg or 2 atm absolute)
• Fresh gas from the flowmeters enters at the fresh gas inlet, passes through a
fixed restrictor and exits the vaporizer gas outlet
57. • Desflurane vapor flows through a variable restrictor to join the fresh gas flow from the
fixed restrictor
• The fresh gas pathway and desflurane vapor pathway are interfaced electronically and
pneumatically through differential pressure transducers, a control electronics system
and a pressure control valve
• When a constant fresh gas flow rate encounters a fixed restrictor, a specific back
pressure that is proportional to the fresh gas flow rate pushes against the diaphragm
of the pressure transducer
• The pressure transducer conveys the pressure difference between the fresh gas
pathway and the vapor pathway to the control electronics system which regulates the
variable pressure control valve so that the pressure in the vapor pathway equals the
pressure in the fresh gas pathway
• This equalized pressure supplying both restrictors is the working pressure and it is
constant at a fixed fresh gas flow rate
• If fresh gas flow rate is increased, more back pressure is exerted on the diaphragm of
the pressure transducer and the vaporizer working pressure increases
60. Datex-Ohmeda Aladin Cassette Vaporizer
• Electronically controlled vaporizer designed to deliver the 5
different commonly used inhaled anesthetics (halothane,
isoflurane, enflurane, sevoflurane, desflurane)
61. Aladin Cassette Vaporizer• color coded and magnetically coded.
• The flow at the outlet of the chamber is controlled by CPU in the
machine.
Benefits:
• Easier to handle
• Automatic record keeping and gas usage calculation.
• Enhanced safety.
• Virtually maintenance free
64. Hazards
1.Overfilling and tilting (tipping)
• liquid agent enters the vaporizer bypass flow
• lethal concentrations of the agent
2. Leaks
• loose filler cap
• awareness
65. Hazards
3.Simultaneous Use of Two Different Vaporizers
• older style vaporizer manifold that holds three vaporizers
• If the center vaporizer is removed, the interlock system becomes
disabled
67. Wrong Agent
• High , low, high method High to low=High
Low to high =Low
Vapor pressures
• Sevoflurane 160
• Enflurane 172
• Isoflurane 240
• Halothane 244
• Desflurane 669
68.
69. Checking Anesthesia Workstation
• A complete anesthesia apparatus checkout procedure must be
performed each day before the anesthesia workstation is first used
• An abbreviated version should be performed before each subsequent
case.
• The preanesthesia machine checkout (PAC) is a checklist-oriented
procedure.
77. References
1. Millers Anesthesia, 8th Edition.
2. Anesthesia equipment principles and applications, 2nd edition, Jan
Ehrenwerth et al.
3. Clinical Anesthesia, Barash.
4. https://www.apsf.org/newsletters/html/2008/spring/05_new_guidelines.ht
m
5. https://www.youtube.com/watch?v=xLu3p40MiaQ&pbjreload=10
6. https://www.ncbi.nlm.nih.gov/pubmed/25060161
7. file:///E:/Guidelines%20&%20Journals/VolatileCostsCalculationsZORA.pdf
Editor's Notes
According to the miller,
Inhalational anaesthetic agents need to be delivered to the lungs for them to work. Of course one cannot simply pour them into the lungs !
,
Most volatile agents exist as liquid at room temperature and atm pressure.
If you take a gas, and compress it really hard, the particles that compose it are brought ever so close to each other. As you keep compressing , the particles will at some point coalesce and convert the gas into liquid. However, if the gas is above a certain temperature, called a "critical temperature", whatever amount of pressure you apply, that gas will not become a liquid. This temperature is called "critical temperature" and every gas has its particular critical temperature.
A gas that is currently below its critical temperature is called a “vapour”. If compressed with enough pressure, it will condense into a liquid.
A gas that is currently above its critical temperature remains a gas. However hard you compress it, it will not condense into a liquid.
Vapourisers have evolved from very basic devices to more complicated ones. Anaesthetists should understand the basic principles of anaesthetic vapouriser, including the principles that affect vapouriser output and how they influence vapouriser design. Anaesthetic vapourisers, used for the administration of volatile anaesthetics, have evolved from the simple masks used for open ether anaesthesia to the present day modern electronically controlled vapourisers designed to deliver potent modern inhalation aesthetic agents.
Fresh gas enters the inlet of the vaporiser and is divided into two flow pathways. The splitting valve, depending on the setting of the control dial, adjusts how much goes through each of the pathways. The fresh gas that is sent along the "by pass" pathway doesn't come into contact with any vapor. On the other hand, the fresh gas that is sent to the vaporising chamber becomes fully saturated with vapor. At the exit end of the vaporiser, the by pass gas (vaporless) meets the chamber gas (fully saturated with vapor) and the two mix. The resultant output depends on how much of fresh gas went though each of the pathways.
When you dial a high anaesthetic concentration requirement, the splitting valve sends more fresh gas via the vaporising chamber.
Similarly, when you dial a low anaesthetic concentration requirement, the splitting valve sends less fresh gas via the vaporising chamber.
Contemporary anesthesia vaporizers for halothane, isoflurane, enflurane and sevoflurane are classified as variable-bypass, flow-over, temperature-compensated, agent-specific, out-of-circuit vaporizers (also called concentration calibrated, automatic plenum, dial-controlled)
Desflurane vaporizer (Ohmeda Tec 6) is of different design
You have seen that the anaesthetic concentration that is output by the vaporiser is determined by the ratio of fresh gas flow that goes through the vaporising chamber and the fresh gas flow that goes through the bypass pathway. This ratio is called the 'splitting ratio'.
ml liquid used/hour= 3 x% x FGF
This is then diluted by an additional measured flow of gases from other (main) flowmeters on the anesthesia machine .
A series of baffles repeatedly redirect the mixed gas flow onto the surface of the liquid anesthetic agent to achieve full saturation.
Certain vaporisers (e.g. "Copper Kettle") use bubbles to increase the surface area for vaporisation. In these, some of the fresh gas flow is bubbled through a disk made out of a special material (sintered disk) that is very porous. The disk is submerged into the anaesthetic agent and when fresh gas is sent through it, a large number of tiny bubbles form. The tiny bubbles of fresh gas have a very large total surface and thus become fully saturated with vapor efficiently.
The anesthetic agent is delivered into the gas stream through a fine nozzle. The rate of delivery depends on the pressure difference P1 to P2 across the nozzle (see figure), and this is adjusted by the throttle valve
If flow through the vaporizer is increased, the pressure across the valve is increased and so more anesthetic is delivered to maintain the same concentration
Therefore, the vaporizer remains accurate despite changes in flow
If an ideal vaporizer existed, with a fixed dial setting its output would be constant regardless of varied flow rates, temperature, backpressure, and carrier gases. Designing such a vaporizer is difficult because as ambient conditions change, the physical properties of gases and vaporizers themselves can change.[53]Contemporary vaporizers approach ideal but still have some limitations
With a fixed dial setting, vaporizer output can vary with the rate of gas flowing through the vaporizer. This variation is particularly notable at the extremes of flow rates. The output of all variable-bypass vaporizers is less than the dial setting at low flow rates (<250 mL/min) because of the relatively high density of volatile inhaled anesthetics. Insufficient turbulence is generated in the vaporizing chamber at low flow rates to upwardly advance the vapor molecules. At extremely high flow rates, such as 15 L/min, the output of most variable-bypass vaporizers is less than the dial setting. This discrepancy is attributed to incomplete mixing and failure to saturate the carrier gas in the vaporizing chamber. In addition, the resistance characteristics of the bypass chamber and the vaporizing chamber can vary as flow increases. These variations can result in decreased vapor output concentration
One method that vaporisers use to increase the efficiency of vaporisation is to dip wicks into the anaesthetic agent. Due to capillary action, the anaesthetic agent rises into the wicks. This dramatically increases the surface area of anaesthetic agent exposed to the fresh gas entering the vaporisation chamber and thereby improves the efficiency of vaporisation.
The carrier gas can be directed using the baffles or spiral tracks that lengthen the gas pathways over the liquid. This increases the time and area of contact.
At lower temperatures, the SVP of volatile agents falls unless the splitting ratio of the gas is altered so that more gas flows through the vaporizing chamber
Because of improvements in design, the output of contemporary temperature-compensated vaporizers is almost linear over a wide range of temperatures.
Modern vaporizers contain a temperature controlled valve which adjusts the splitting ratio .
To maintain a constant output from the vaporizer, mechanisms to compensate for fluctuations in temperature are to be employed.
The falling temperature (lowering energy) of the liquid means that less molecules are able to escape. i.e. as vaporisation happens, the temperature of the liquid falls causing less vaporisation.For vaporisation to occur, the anaesthetic molecules have to "escape" from the liquid state and become vapor. This process reduces the 'energy' left in the remaining liquid.
As more and more molecules escape, more and more energy is lost from the liquid. The temperature of a liquid is a measurement of how much 'heat energy' the liquid has. Therefore, as the escaping molecules reduce the energy left in the liquid, the temperature of the liquid falls.
The automatic temperature compensating valve uses the physical property that substances (e.g. metals and liquids ) become smaller when the temperature lowers. A metal rod (shown in black below) shortens as the temperature drops. Similarly, a liquid filled in collapsing bellows (shown in green below) becomes smaller in volume when cooled to a lower temperature.
In the design that uses a metal rod, the rod offers some resistance to flow into the vaporising chamber. As the vaporiser cools, the rod becomes shorter, making the valve move away from the opening. This reduces the resistance to flow and thus more flow occurs into the vaporising chamber.
In a bimetallic strip, two metals with very different degrees of thermal expansion .In the example below, the "green" metal expands and contracts less than the "red" metal.
When the temperature of the vaporising chamber drops, the bimetallic bends and moves away. This reduces the resistance to flow and thus more flow occurs into the vaporising chamber.
may affect the proportion of total flow passing through the vaporization chamber.
When carrier gas is switched from O2 to N2O, there is a rapid transient decrease in vaporizer output followed by a slow increase to a new steady-state value
This transient decrease in output is due to N2O being more soluble than O2 in halogenated liquid. Therefore the quantity of gas leaving the vaporizing chamber is transiently diminished until the anesthetic liquid is totally saturated with N2O
When the bag is squeezed (positive pressure ventilation), pressure is transmitted back into the vaporiser as shown below. This "back pressure" is transmitted to both, the "by pass" channel and also to the vaporising chamber. This "back pressure" opposes the flow of the fresh gas in both the "by pass" channel and the vaporising chamber. The fresh gas entering the vaporiser tries to move forward and gets compressed both in the 'by pass' channel and the vaporising chamber. However, the vaporising chamber volume is much larger than the 'by pass' channel volume, and thus, more fresh gas gets compressed into it than into the 'by pass' channel.
when the positive pressure is suddenly released (expiration). The previously compressed gases now suddenly expands in all directions.
Some of the rapidly expanding gas (containing vapor) enter the inlet of the vaporizer and cross over into the 'by
Greatest increase in pumping effect occurs at:-
1. Low fresh gas flow rates
2. Low concentration dial settings
3. Small amount of liquid agent in vaporizer sump
4. Large and rapid changes in pressure
Keep the vaporizing chamber small or increasing the size of bypass chamber.
LONG INLET TUBE: the extra gas containing vapor expands into the long inlet tube and doesn't reach the 'by pass' channel.
The high-resistance pathway through the vaporizing chamber offers less resistance under hypobaric conditions, increasing vaporizer output
The delivered partial pressure and volume % increases.
Closer the vapor pressure is to the barometric pressure, greater the effect.
The clinical effect is unchanged
Desflurane has a very low boiling point (about 23 degrees Centigrade) and even at room temperature, has an high vapor pressure.
Desflurane is an extremely volatile anesthetic with a SVP of 669 mmHg at 20C and boiling point of 22.8C
These properties make it challenging when it comes to vaporization and production of controlled concentrations of vapor
If conventional variable bypass vaporizers are filled with desflurane, it will boil at 22.8C in the vaporizing chamber leading to uncontrolled output from the vaporizer, and overdosing
Liquid desflurane in heated is a chamber (the sump) to 39C to produce vapor under pressure ( 1500 mmHg or 2 atm absolute)
Fresh gas from the flowmeters enters at the fresh gas inlet, passes through a fixed restrictor and exits the vaporizer gas outlet
Desflurane vapor flows through a variable restrictor to join the fresh gas flow from the fixed restrictor
The fresh gas pathway and desflurane vapor pathway are interfaced electronically and pneumatically through differential pressure transducers, a control electronics system and a pressure control valve
When a constant fresh gas flow rate encounters a fixed restrictor, a specific back pressure that is proportional to the fresh gas flow rate pushes against the diaphragm of the pressure transducer
The pressure transducer conveys the pressure difference between the fresh gas pathway and the vapor pathway to the control electronics system which regulates the variable pressure control valve so that the pressure in the vapor pathway equals the pressure in the fresh gas pathway
This equalized pressure supplying both restrictors is the working pressure and it is constant at a fixed fresh gas flow rate
If fresh gas flow rate is increased, more back pressure is exerted on the diaphragm of the pressure transducer and the vaporizer working pressure increases
The heart of the vaporizer is the electronically flow control valve located in the vaporizing chamber’s outlet. This valve is controlled by a central processing unit (CPU)
Consists of a permanent internal control unit housed within the machine and an interchangeable Aladin agent cassette which contains anesthetic liquid
Aladin agent cassettes are color coded for each agent
Also magnetically coded so that the machine can identify which anesthetic cassette has been inserted.
A fixed restrictor is located in the bypass chamber
Flow measurement units are located in the bypass chamber (FBC) and in the outlet of the vaporizing chamber (FVC)
Machine accepts only one cassette at a time and recognizes cassette through magnetic labelling.
: can be handled or stored in any position, weigh only 2-3 kgs.
It is important to fill the correct agent into the correct vaporiser. If a wrong agent is filled into a vaporiser, you will be giving the wrong drug, and worse, since vaporiser designs for different agents vary, you may seriously overdose your patient.
Early vaporisers had simply a funnel into which you could pour virtually anything by mistake (including coffee).
Modern vaporisers have special filling systems specific for each anaesthetic agent to prevent inadvertent filling with an wrong agent. Think of it as a "lock and key" system, i.e. a particular key will fit only a specific lock.
thereby preventing administration of more than one inhaled agent at the same time
2. Overfilling is minimized because filler port is located at the maximum safe liquid level
If this occurs, the vaporizer should be purged with a 1.high flow rate of oxygen (10 L/min) from the machine flowmeter and with the vaporizer concentration dial set to the maximum concentration setting
2.
Leaks can also occur at the O-ring junction between the vaporizer and its manifold
Vaporizer topped-off with incorrect agent
Vaporizer output is less easily predicted, and large errors in vapor administration can occur
More likely with vaporizers not equipped with keyed fillers
